The present invention relates to a filter body for retaining particles from a gas flow flowing through the filter body, including flow paths for the gas flow. The filter body is heat-resistant and regeneratable. The invention also relates to a process for the production of a heat-resistant and regeneratable filter body which serves for retaining particles, in particular soot particles, from a gas flow flowing through the filter body, preferably from an internal combustion engine. The filter body which is produced in that way also has predetermined flow paths.
A preferred area of use of the invention is an application thereof in the field of exhaust gas filtering in relation to internal combustion engines. Environmental awareness which is ever increasingly growing world-wide has resulted in endeavors being made, particularly in that area, to find ways of reducing, cleaning and neutralizing emissions and ultimately reducing them to a potential which is as low as possible for mankind and its environment. In the middle of the Eighties, for example, that directed attention to soot particles in relation to motor vehicles. Various filter structures were also produced for being able to filter such particles out of exhaust gases. Set out below is an overview of some filter installations which are known in the state of the art and to the structural configurations, materials and disclosure of which the invention also has recourse.
German Patent DE 37 44 265 C2 discloses a soot filter for exhaust purification in connection with internal combustion engines, which has corrugation configurations and fold configurations for changing direction of exhaust gas in the soot filter. Flat filter material is integrated into the layers of the soot filter. A production process for introducing corrugation configurations and fold configurations is also described, wherein bar portions extending transversely at spacings are introduced into steel sheets being used. German Published, Non-Prosecuted Patent Application DE 33 30 020 A1 discloses a Diesel exhaust gas filter including wire mesh having sieve mesh configurations that are illustrated in many different forms. Respectively open and closed end surface portions are disposed in a mutually opposite relationship, thereby predetermining the flow path in that Diesel exhaust gas filter. The closed end surface portions are produced by squashing components of the Diesel exhaust gas filter. European Patent 0 134 002 B1 shows configurations which are possible in that respect and discloses a production process for winding a sieve mesh with a cover layer to form a gas-permeable assembly. A further document, German Published, Non-Prosecuted Patent Application DE 29 51 316 A1, describes a catalytic filter for Diesel exhaust gas purification, which includes a metal sieve mesh configuration with alternate layers of corrugated sieve mesh and a closed cover layer. The surfaces of the filter are closed by a cover device in such a way that a closed end surface portion is disposed opposite an open end surface portion. The filter action of that catalytic filter is achieved by way of the mesh width and by way of the porosity of an oxide layer to be applied to the sieve mesh. Another technical way of producing flow paths in a filter body is illustrated in German Published, Non-Prosecuted Patent Application 27 33 640. A carrier matrix is described for a catalytic reactor for exhaust gas purification in connection with internal combustion engines, with coated steel surfaces. Raised portions and recessed portions in a steel sheet co-operate in form-locking relationship with an adjacent steel sheet whereby flow paths are formed in the carrier matrix. German Patent DE 37 44 265 C2 again discloses a soot filter, wherein layers of corrugated or folded material are closed transversely with respect to the longitudinal direction of the corrugation or fold.
Besides the filters which have just been described above and which are made from metal sheets or foils, the state of the art also includes extruded or ceramic filters. German Utility Model G 87 00 787.8 discloses a soot filter for Diesel engines, wherein ceramic fibers which are disposed in a random or irregular array form an open-pore filter body. Embedded in the filter body is a heating wire which is intended to heat substantially the entire volume of the filter body to a temperature. Patent Abstracts of Japan 57-163112 discloses a soot filter in which ceramic foam is surrounded on both sides by metal foils. The metal foils can be heated with electrical energy. The ceramic foam with the metal foils is wound or coiled to form a filter body. German Patent DE 35 01 182 C2 discloses an exhaust gas filter for Diesel engines, which includes a monolithic filter block of porous ceramic. That filter block has a multiplicity of passages extending in the main flow direction of the exhaust gas. The passages are closed alternately by plugs which are introduced into the same. The plugs are intended to cause the exhaust gas to flow through more than one passage wall in succession. In that configuration the porosity of the passage walls which are connected in succession by the operative plugs is intended to decrease in the through-flow direction. The reduction in porosity is intended to be achieved by applying a porosity-reducing agent to a filter block blank, wherein multiple application is intended to produce different zones of varying porosity over the exhaust gas filter produced in that way. That exhaust gas filter is very expensive to produce due to the necessary plugs and the wall coating which is required a plurality of times under some circumstances. Division of the monolithic filter block into zones of different wall coating porosity is intended to considerably reduce a back pressure upstream of the filter, which increases with increasing operating life. The configuration of the flow through the filter block, which is compelled by virtue of its structure, admittedly ensures that the gases flow through successively disposed porosity zones. However, it is not possible to gather information about the precise flow path. In addition, individual gas flows are repeatedly mixed together in the filter block and due to the effects which occur in that situation, they produce an undesirable pressure drop in the filter block itself.
It is accordingly an object of the invention to provide a heat-resistant and regeneratable filter body with flow paths and a process for producing the filter body, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and processes of this general type, in which the body combines a high level of filter capacity with low pressure drops in a filtration effect and in terms of through-flow, production thereof is to be possible in a small number of working steps and the process permits a particularly labor-saving structure with respect to filter bodies.
With the foregoing and other objects in view there is provided, in accordance with the invention, a heat-resistant and regeneratable filter body for retaining particles from a gas flow flowing through the filter body, comprising mutually separate flow paths for a gas flow; and at least one respective first filter stage and one respective second finer filter stage disposed in succession in flow direction in each of the flow paths.
Separation of the flow paths ensures on one hand that a predetermined defined path through the individual filter stages is followed by each partial gas flow. On the other hand, pressure losses by virtue of mixing phenomena with respect to different partial gas flows are prevented. Separation of the individual flow paths also facilitates the structural configuration of the filter body. The respective filter stages in a flow path are then disposed independently of adjacent flow paths.
The first filter stage retains particles which are of a certain minimum size from the gas flow flowing therethrough. It therefore serves as a kind of coarse filter which prevents the larger impurities that are present in terms of surface area and/or volume from flowing further through the filter. The gas flow which is thus filtered for the first time is then taken to a second finer filter stage. The latter is then capable of filtering particles out of the gas flow which are smaller in comparison with those which were retained at the first filter stage. Dividing up the filtering action through the filter body into different filter stages has the advantage, as considered over a filter surface of a filter stage, that there are always sufficient intermediate spaces through which particles that are even finer than the particles to be filtered out in that filter stage can flow through the same. If there were only one filter stage with a single, predetermined maximum permissible permeability, under some circumstances a much too great pressure drop would be found to occur, in the event of all filtered-out particles accumulating. The use of filter stages which are disposed in succession in a flow path means that the loading with particles is also distributed over a plurality of filter surfaces, while the overall loading due to particles is also distributed to the individual, mutually separated flow paths. In operation, corresponding to the magnitude of the filter surfaces of the filter stages, when added together, the configuration affords a lower pressure drop in comparison with a filter involving a single level of porosity if the flow paths for the filter surfaces which are added together have an advantageous structural configuration in terms of flow dynamics.
In many uses it is sufficient for the flow body to have two filter stages. However, in accordance with another feature of the invention, depending on the area of use of the invention, the nature of the particles and the particle loading in the gas flow, under some circumstances it is more desirable for three or more filter stages with the sizes of the filter openings decreasing in the flow direction to be disposed in the filter body. In that way it is possible for the filtration process to be implemented in the filter body, in a very finely controlled and metered manner. If it is known, for example, that there is primarily a given particle size in the gas flow to be filtered, and that particle size has a certain width of dispersion with respect to its dimensions, the use of a plurality of filter stages which are very close together in regard to their filter opening sizes provides for distribution of the filtration effect to a plurality of filter stages. Overloading of an individual filter stage in that way is eliminated from the outset.
In accordance with a further feature of the invention, the heat-resistant and regeneratable filter body has alternate layers of filter material and layers of gas-impermeable material. The layers of filter material have a filter opening size which decreases in the flow direction while the layers of gas-impermeable material form flow guide surfaces. The flow guide surfaces deflect the gas flow two or more times so that it has to cross the layers of filter material two or more times. While the flow guide surfaces serve to form the flow paths and the mutual separation thereof, the layers of filter material predominantly form the respective filter stages. The filter material can be, for example, a sieve mesh, as well as a braid or other known heat-resistant filter substances. For example, they may be fiber filters or ceramic foam filters. An example of the latter would be polyurethane foams which are impregnated with filter material (Corderite or A 1203), then dried and then calcined. Fiber filters in turn are heavily dependent in their effect on the interplay of the fiber diameters with the particle diameter. In order to provide the filtration of Diesel particles in motor vehicle exhaust gases with a particle size of between 0.1 and 1 xcexcm diameter, fiber diameters of about 4 to 30 xcexcm are highly effective. In order to achieve adequate stability, in particular in regard to regeneratability, the fiber diameters can also be selected to be larger, in dependence on the material involved. Moreover it is also possible to use steel wool filters, having a wire diameter which is, for example, 0.25 mm. The layers of fiber material and gas-impermeable material can be formed of metal but they can equally also be of ceramic material. Limitations in regard to the choice of material arise at most by virtue of the conditions under which the filter body is used.
In accordance with an added feature of the invention, in an advantageous area of use, involving retaining particles from internal combustion engines, in particular soot particles from Diesel engines, the temperature development in regeneration of the filter body represents a limit in regard to filter body materials, like the temperature of the gas flow itself when flowing through the filter body. With a catalytic non-precious metal coating, it is possible to reduce the soot ignition temperature from about 500xc2x0 C. to about 400xc2x0 C. The use of metallic fuel additives makes it possible to even reduce the ignition temperature to 150xc2x0 C. It is to be noted, however, that, for example, when travelling, in the event of the soot burning off, under adverse conditions temperatures of 1400xc2x0 C. and more could also occur. However, they are prevented by a configuration of a filter body, in accordance with the invention. In regard to other uses of the filter body, the limitations in regard to the materials being used arise, for example, by virtue of necessary acid resistance, the erosion which occurs by virtue of the nature of the particles, and their flow rate or other influencing parameters.
However, the costs of the structural configuration of the flow paths are also dependent on the area of use of the filter body. The flow paths may be provided by gaps between layers and, as in an embodiment of the invention, by virtue of mutually spaced-apart walls, for example of different extruded circular sleeves, cases or tubes at different spacings, which when fitted one into the other provide a filter body. If the filter body is subjected from the exterior to the effect of the gas flow to be filtered, then the filtered gas flow can be discharged from the interior of the last extruded tube member. The sleeves, cases or tubes can be connected together, for example by a support device. Another possible way of holding those tubes, each of which act as a filter stage, involves supporting them at their end surfaces.
In accordance with an additional feature of the invention, in the event of a radial flow through the filter body, the end surfaces must be closed in order to form the flow paths from the exterior inwardly of such a filter. Advantageously, the permeability of the filter stages also decreases in that direction. That through-flow direction has the advantage of ensuring that the largest particles are also caught at the outermost tube member or case which naturally has the largest surface area. However, by virtue of the size of the surface area of the tube, the fact that the large particles are retained does not result in the pressure drops being as high as would occur at, for example, the innermost tube, with the same gas and particle flow acting thereon.
In accordance with yet another feature of the invention, another structural configuration of the flow paths provides the same in the form of flow passages. Accordingly, the filter body can be wound or coiled, layered, extruded or produced in any other manner. In particular, structures which are to be considered as a flow passage are configurations of flow guide surfaces which form and/or subdivide a cross-section through which the gas flow is to pass and which is smaller than other dimensions of the filter body.
In one structural configuration the filter stages again are formed from individual filters while in another structural configuration the filter stages are also formed from a plurality of filters as a layered configuration.
In accordance with yet a further feature of the invention, the filter body has a stacked and/or wound layer with a metal foil, with a first and/or second filter being incorporated into the layer. In that way the layer can be shaped as desired, while in the case of a filter body it can also contribute to the stability thereof. The integration of one or two filters into a layer with a metal foil also makes it possible to use materials for the filter which in themselves do not have an adequate level of strength or a specific structural configuration, for example loose fillings or the like. In that configuration a filter is associated with a filter stage.
In accordance with yet an added feature of the invention, the configuration of a filter stage or a plurality of successive filter stages provides that the first and the second filter form an interconnected or coherent filter. That can mean that the interconnected filter extends over a plurality of flow paths or that the filter stages which are disposed in succession are interconnected by the filter itself. The filter can be, for example, a metal mesh, in which case the mesh spacing of the metal mesh varies over its length and thereby provides different filter stages. It is, however, also possible to consider other filters which are known from the above-mentioned state of the art.
In accordance with yet an additional feature of the invention, in order to make particularly effective use of a filter stage or a filter, it is disposed in a plane in a flow passage in such a way that a cross-sectional area of the filter stage in the flow passage, which can be acted upon with gas to be purified, is larger than the smallest cross-sectional area through that flow passage. Distribution of the gas flow over a larger surface area when passing through the filter stage in comparison with the cross-sectional area of the passage, on one hand avoids major pressure drops since more openings are made available for the gas flow therethrough in a filter stage. On the other hand, that configuration makes it more difficult for those openings to become blocked by virtue of the increased number in comparison with a filter stage disposed perpendicularly in the cross-section. Furthermore, in that way the filter or the filter stage is also structurally useful. In particular, it can be disposed in such a way that it makes a contribution to the stability of the filter body just as to the elasticity thereof. In an embodiment of a filter body the filter stage is therefore deformable under loading. That deformability may be plastic but it may equally also be elastic, depending on the loadings which are to be envisaged in the appropriate area of use of the filter body.
In accordance with again another feature of the invention, the filter body has a filter stage with a trap at which filtered particles of that filter stage preferably accumulate. The term trap is used in this case on one hand to denote a spatial configuration in which, by virtue of the flow through the filter stage, a suitable geometry thereof permits a migration movement of the retained particles at the filter stage. The migration movement can be guided by suitable construction of the filter stage. For that purpose the filter stage may have recesses, grooves, constrictions, grid-like devices as well as guide surfaces. On the other hand, the term trap is used to denote all those devices which at a filter stage ensure that there is a virtual main point of attraction for the particles that are retained. They may be of a chemical, physical or electrical nature.
In accordance with again a further feature of the invention, accumulations of particles in and around the trap, which occur at the filter stages when gases flow through the filter body for a prolonged period of time, facilitate regeneratability of the filter body. When the filter body is in use over a long period of time, its filter effect decreases, as considered in relation to time. An attempt must therefore be made to at least approximately regain the original filter action.
In accordance with again an added feature of the invention, the filter body has measures for regeneration of the filter stage, at least adjacent the trap. The regeneration device may be a device for thermal conversion of the particles which have accumulated there. However, other steps such as, for example, discharge flow passages for the particles or the like are also possible. Regeneration of the filter body can be effected chemically, thermally or mechanically, while the selected mode of regeneration generally depends on a number of parameters. A decision is to be made as to whether mechanical removal, for example by shaking the particles out or by floating them off, is more advantageous than other possible regeneration modes, depending on the respective structure of the filter body, the materials used in relation thereto, and the installation in which the filter body is disposed. It will be appreciated that the nature of the particles retained and their behavior, for example whether they cake together or the like, accordingly also plays a part. It may also be advantageous if different filter stages of the filter body can also be regenerated differently. For example, in the case of filter stages which are downstream in the flow direction, it may be desirable for the particles which are particularly fine there to be thermally converted while the coarser particles at the filter stages which are more upstream in the flow direction would be better disposed of by mechanical measures. The choice of regeneration procedure is therefore also dependent on energy aspects.
In accordance with again an additional feature of the invention, the filter body at least partially has a catalytically active coating. This can serve for conversion of the gas flow flowing through the filter body, but it can also serve a function for the filter body itself. This can be an increase in the temperature of a part of the filter body or the entire filter body by virtue of the catalytic reaction, as well as possible regeneration of a filter stage. The filter body can be used in chemical installations just as in installations dealing with exhaust or waste gases, in which the gas flow is at such high temperatures that filter bodies which are not heat-resistant would be damaged.
In accordance with still another feature of the invention, in the case of a use in connection with Diesel motor vehicles, it is particularly advantageous to provide two different catalytically active coatings in the honeycomb body in order to specifically and deliberately improve certain properties. Thus the conversion of nitrogen oxides in the exhaust gas into harmless components is possible at its most effective when there are hydrocarbons which can bind the oxygen to be reduced by the nitrogen oxide.
In accordance with still a further feature of the invention, the first stage of the filter body is provided with a coating which promotes the reduction effect, for splitting up nitrogen oxides. It is only thereafter that there follows a coating for promoting oxidation of the hydrocarbons which have remained (and carbon monoxide, if present).
In accordance with still an added feature of the invention, at the same time or alternatively it is possible to provide the various layers with different coatings, in particular the gas-permeable layers in which soot accumulates with a catalytically active coating that reduces the soot ignition temperature, and the gas-impermeable layers with a coating for promoting the oxidation of hydrocarbons.
In accordance with still an additional feature of the invention, the filter body is heatable. This can be effected electrically but also in other ways, for example by a chemical action, by heat transfer or the like.
In accordance with another feature of the invention, the first and/or the second filter stage is directly heatable. On one hand, that makes it possible to heat the gas flow while on the other hand, the filter stage can also be regenerated in that way. Heating of the filter body can therefore involve the entire body, but it may equally also involve only certain parts thereof. That can be controlled by the structure of the filter body and, for example, by virtue of the electrical connections, when using an electrical heating configuration. In particular, the filter body can be constructed in such a way that it has surfaces which heat up particularly vigorously while in comparison other surfaces have only a certain degree of heating action.
With the objects of the invention in view there is also provided a process for the production of a heat-resistant and regeneratable filter body for retaining particles, in particular soot particles, from a gas flow flowing through the filter body, preferably from an internal combustion engine, which comprises producing predetermined flow paths while simultaneously disposing at least a respective first filter stage and a respective second finer filter stage in each flow path; and placing the first and second filter stages in succession along the flow paths in a flow direction through the filter body.
In accordance with another mode of the invention, such a process presents itself in particular for a filter body with a first and a second filter stage, as has already been described above. The process enjoys the advantage of elimination a subsequent filter stage which is to be fitted into the filter body after production thereof, as a working step in the process. As a result the process not only enjoys a time saving in comparison with the previous processes in the state of the art, but it also enjoys cost advantages in comparison therewith. Therefore, in accordance with the invention the term xe2x80x9csimultaneouslyxe2x80x9d is used also to denote the working step in which a flow path is produced, for example in the case of sheet metal layers by applying various ones thereof or when winding or coiling the same.
In accordance with a concomitant mode of the invention, if the configuration involves an extruded body, the process can be implemented in such a way that, upon extrusion of the filter body, its porosity is attained by changing or selecting a porosity of an extrusion material in accordance with a desired porosity of a filter stage. That can be achieved by virtue of different dimensions with respect to the extrusion material or materials being used. When dealing with slurries, they are, for example, respectively mixed with different porosities. If a filter body is to be sintered, it is possible in this case for the sintering mold to be filled with different sintering material sizes, in accordance with the desired porosity of a filter stage. Suitable apparatuses for the production of a filter body of that kind advantageously have mixing devices, which permit a change in the sizes of material used in the production of the filter body. Depending on the filter stage desired, the process can be implemented in such a way that on one hand there is a steady transition from one porosity to the other, while on the other hand the process also permits a specific demarcation between two different porosities.
Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in a heat-resistant and regeneratable filter body with flow paths and a process for producing the filter body, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.